External rendering - Backgrounds

The background to which the render is applied will, to a large extent, determine the nature and strength of the render. To ensure a good bond, three factors must be considered: the suction of the background (i.e. its ability to absorb moisture), the strength of the render and the mechanical key of the background. If the suction is too low the render will not bond properly to the background and if too high the background will suck the moisture out of the render, preventing proper hydration.

Dense smooth materials

Smooth concrete and dense smooth bricks with flush joints do not provide a suitable background for render, unless some form of mechanical key is provided. One of the best methods is to use a stipple coat, comprising one part cement and two parts sand with a proprietary bonding agent, such as PVA. This is mixed with water and then brushed generously over the surface. Stippling with a coarse brush gives a rough textured surface which, once thoroughly dry, will provide a mechanical key and improved suction for the render.

Moderately strong, porous materials

The majority of bricks and non-aerated blocks usually have sufficient suction and provide an adequate mechanical key for most renders. Mortar joints should be raked out and the wall slightly dampened before applying the render – this helps to reduce suction. Alternatively, some builders prefer to use a stipple (spatterdash) coat based on sand and cement, and possibly containing proprietary bonding agents.

Weak, porous materials

These backgrounds, such as old porous brickwork and rubble stonework, may have very high suction. This can be overcome by dampening the wall or by the provision of a stipple coat (see image below). If excessive suction is likely, a water retention admixture can prevent rapid drying of the render and enable it to be worked satisfactorily.

Metal lathing

Some surfaces require separate support for the render, usually in the form of metal lathing. Examples include old crumbling brickwork (see photo below), porous stonework and some lightweight blocks. Metal lath can also be used where render has to be applied to composite surfaces, such as a concrete frame with block infill. If lath is not used there is a danger of cracking, caused by differential movement of the two background materials. The metal lathing, which can be galvanised or stainless steel, can be fixed directly to the surface using masonry nails or stainless steel screws. Three render coats are necessary, the first of which protects the render from corrosion and forms a key for subsequent coats.